"* �I Q~DfOnT r'nCUMENTATION PAGE I ,j. ,I

AD A263 130 .... .. . * ....... ..[..19 March 1993 Technical

4. IIILt ANLJ luoitiuc5 ~t4fG .Ash

Scanning Probe Lithography. 1. Scanning Tunneling Microscope- N(XX14-91 91-Induced Lithography of Self-Assembled n-Alkancthiol MonolayerResists

6. AUTHOR($)

Claudia B. Ross, Li Sun, andRichard M. Crooks

PERFORMING ORGANIZAT1ON NAME(S) AND ALUUMSS[ES) i PIW{kf ONtANt,.A ft

Department of Chemistry 5University of New Mexico

P% Albuquerque, NM 87131

00 *SPONSORING,&MONITORING AGENCY NAME($) ANU DOE I

0 ~ ~~Office of Naval ResearchCI GN A1,01funt

800 North Quincy StreetArlington, VA 22217-5000

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Prepared for Publication in Langmuir 93 4 (, - )

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.13. ABSTRACT (Mjxtmum 200 w0 CIs)

The tip of a scanning tunneling microscope was used to fabricate geometrically well-defined

structures within organized, self-assembled monolayer resists that have critical dimensions ranging

from 60 nm to 5 pim. To achieve nanometer-scale lithography, a Au( Il1) substrate was coated

with a self-assembled monolayer of HS(CH2)17CH3. which functions as an ultrathin (-2.5 nm)

resist, and then the resist was etched by an STM tiq. This treatment results in window-like features

that penetrate the organic monolayer. Nanolithographically defined features have been

characterized by scanning tunneling microscopy, scanning electron microscopy, and

electrochemical methods. For example, since mass and electron transfer to the conductive Au

substrate are blocked by the monolayer everywhere except in the STM-etched regions, the

windows serve as ultramicroelectrodes. The limiting current that results from radial diffusion of a

bulk-phase redox species to the etched window is in close agereement with that predicted bv.theorvy14. SUBJECT TERMS 15. NUMBER OF PAGES

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Technical Report No. 5

Scanning Probe Lithography. I. Scanning Tunneling Microscope- idu'cd Lithography ofSelf-Assembled n-Alkanethiol Monolayer Resists

by

Claudia B. Ross, Li Sun, and Richard M. Crooks

Prepared for Publication 41..

in

Langmuir

Department of Chemistry

University of New MexicoAlbuquerque, NM 87131 D t

March 19. 1993

Reproduction in whole or in part is permitted for any purpose of the United States Government.

This document has been approved for public release and sale;its distribution is unlimited.

Technical Report 5,

Scanning Probe Lithography. 1. Scanning Tunneling

Microscope-Induced Lithography of Self-Assembled n-

Alkanethiol Monolayer Resists

Claudia B. Ross, Li Sun, and Richard M. Crooks*

Prepared for publication in Langmuir

The tip of a scanning tunneling microscope was used to

fabricate geometrically well-defined structures within organized.

self-assembled monolayer resists that have critical dimensions

ranging from 60 nm to 5 tm. To achieve nanometer-scale

lithography, a Au(lll) substrate was coated with a self-asspmbled

monolayer of HS(CH2 )1 TCH 3 , which functions as an ultrathin (-2.5

nm) resist, and then the resist was etched by an STM tip. This

treatment results in window-like features that penetrate the

organic monolayer. Nanolithographically defined features have

been characterized by scanning tunneling microscopy, scanning

electron microscopy, and electrochemical methods. For example,

since mass and electron transfer to the conductive Au substrate

are blocked by the monolayer everywhere except in the STM-etched

regions, the windows serve as ultramicroelectrodes. The limiting

current that results from radial diffusion of a bulk-phase redox

species to the etched window is in close agreement with that

predicted by theory.

The tip of a scanninq ýuntI,, JVt

fabricare geometrically "at-' I ý -At , f

self -assembled monolayer, r e , ..

ranging from 60 nm to 5 m.

lithography, a Au(wl!! b .,

monolayer of HS(CH 2ý ) j17CH 3 . which f'r, C t

nfn) resist, and then 'he r. . ' b L #..

treatment results in ;i'dow -1 .u," 1 o

organic monolayer. Na h . , Ly ,t,0: .Z j P. I ,

been characterized by scar, ncjng u 'rOe ;

electron microscopy, and e I ec r hemi\d :n I re

since mass and electron transfer to the onJ' u ' ....e r a..

are blocked by the monolaver everywhere ex..p...' in..

regions, the windows serve as uerami .... .. nq-

current that results from radial diffusion of a bulk-phase redo:x

species to the etched window is in close agreement with that

predicted by theory.

-2-

we wish to report t e. f rz~ --x A!fp Keso s

induced lithography of orginized' .3y.

experiment, resists cn ) of , 1y•s 1 ,3-M', 1%-e

alkanethiols which, wh,-i .A, :,

approximately 2.5 nm-thick barrl t :r to 1' ss1 t•d :?'j .i

transfer.2- 4 When a scanning tunneling , 4

positioned near the Au substr7te and t okste •e <it t'ý2c,

it induces desorption of the monolayer resis:

by STM, electrochemical methods, aind scannin.j eiect m ,ry

confirm that this technique is usefu. for flib-catn

geometrically well-defined, nanoimete -.sca a Stt e h UU

ultramicroelectrodes.

Most lithographic processes rely on light-induc r

transformations of organic polymers for fabrication micrlo'-

scale surface features. Present commercial photolithographic

processes are wavelength-limited to about 0.5 pm critical

dimensions, 5 but the development of new technologies that can

reduce this limit to 0.1 - 0.2 pm is essential for high--density,

high-speed microelectronic applications. The present sLate-of-

the-art for lithographically-defined surface features is quantum

dot cylinders with diameters of about 100 nm and thicknesses

around 10 nm. 6 The thickness is limited by the quaity of

iuecular beam epitaxy-deposited thin films, and the lateral

dimenizion is defined by the rebuLucion of electron beam

lithography. In principle, electron beam lithography should only

-3-

be limited by the wavelencgh ot . ,. *n .2 ,,., ;.. .J..--. ::.

however, as a result D) torw, scirt•.: ':-

resist and backscattering tr-m the fu-ra 1 -,:1" ,'-1,u T w

critical dimensions less than 101 .1 -ao Lt- I t 11,.

Because of the physical -, . •

feel it is desirabie t .v,..)- . ::.-x,-

fabricating structures wih cu r.: eis i: . :,,.

200 nm.

Scanning probe devi ce%-e r i: s deeope Jy L :%g ,

in 1982," and since that time have been "-:u p! z:arl 'y a o

obtaining topographical and electronic surface maps. Hcwever,

they can also be used to directly modify the cheph-ca or p}hsI

structure of surfaces. 8 In this paper, we present -he first

example of deliberate STI etching of an organic monolayer reslst.

to produce well-defined features with critical dimensions as sma~l

as 60 nm.9 Since it has been clearly demonstrated that

interpretation of STM images is sometimes ambiguous0 i0 we also

provide an independent electrochemical analysis of some of the

larger lithographically-defined features.

EXPERIMENTTAL SECTION

Substrate Preparation. A 0.25 mm-diameter Au wire (99.998%) was

cleaned by dipping in freshly prepared "piranha solution" (3:1

conc. H2 SO 4 -30% H20 2 , Caution: piranha solution reacts violently

with organic compounds, and it should not be stored in closed

-4-

containers). Au(lll) surfac s ..- . p sv . :'

in a H2 /0 2 flame under N2 , and t.hen an:, .::

the flame. 1 1 - 1 3 This treatment •esults 'n uppco×.: tKt , :f-

diameter spheres that contain j few Auli1` facets,.

The facets are typically e!lipticail, with of lon1 ,"iX" of a<bUt Y,

pm, and are composed of atomically flat terrIce.z ,b run

wide. The Au balls were cleanej again in r:

then rinsed with ethanol. To facilitate ecectrochcaIc

experiments, the entir e bal,; excec• !in nf s

Au(lll) facet, was covered with silicone ruhbbh-r uDo-Qsrnnin

Catalog No. 698). Prior to monolayer adsorption, the exposed

facet was usually polished electrochemically in an aqueous 0.1 M

HClO4 /5x10- 5 M HCI solution: this process eliminates adsonrbed

organic material from the Au surface and tends to reduce the

number of Au surface defects. 1 3 , 14 Cyclic volturnmetry co firmed

the presence of a clean Au(ll1) surface. The freshly prepared

surface was immersed in a 1 mM ethanolic solution of

octadecylmercaptan, HS(CH2 )1 7 CH 3 , for 24 h, removed, rinsed, and

then attached to a home-built STM-substrate holder for subsequent

etching and analysis.

Scanning Tunneling Microscope Etching and Imaging. A Nanoscope iI

STM (Digital Instruments, Santa Barbara, CA) was used for all ST,7

experiments. Images were obtained using a bias voltage of +300 mV

and tip currents in the range 0.10 to 0.11 nA (scan rate = 1.34

Hz). Positive bias voltages indicate that electrons tunnel from

the STM tip to the Au substrate. Tips were mechanically cut from

--5--

l b| Cth ngC~ l la

Pt /Ir (80 /2 0% ) w i r-e . T h e e- t c h Ing C, 0 UI li 1 , ....... ..... .

of the nanolithographicaaly-def med eae,, r, .md ý , e... _e_

discussed in the text. The S,!M Z-piezo was ca lbt:ttd hy

measuring several independently prepared Au2•1.. ) :....... c'. stet

edges and correlating the mean experimental•. value rotl..

theoretical Au(1lll interlayer spacinq of 0.235 -um.

Electrochemistry. Electrochemical experimueýnts we car'•ed out

a single-compartment, three-electode gass cell c..-... ,ai'ng a

Ag/AgCI, KCI(sat'd) reference electrode and a Pt co nter

electrode. Cyclic voltammograms were obtained in purifred (Mi.i!-

Q, Millipore), deoxygenated aqueous solutions ccn '~ing of 5

Ru(NH3)6 3 + and 0.1 M KCI.

RESULTS AND DISCUSSICN

STM images of a stepped Au(lll) surface covered with a single

monolayer of HS(CH2) 1 7CH 3 before and after intentional probe

etching are shown in Figure 1. The top 400 x 400 nm image was

Figure 1

obtained with a +300 mV bias voltage and a tunneling current of

0.10 nA. The Au surface contains primarily 100-200-nm-wide

atomically flat terraces separated by monoatomic steps. Small

circular defect structures, typically 5 nm in diameter, always

-6-

appear homogeneously distributed within the - H$xH-){O]

monolayer.9b0iA We are studying these sctures, Z<. ut .

present time we are uncertain of thejr precie 5t:7Ct.r .

origin. Immediately after obtaninng the top Image, h,'. .- 1

x 100 nm region was scanned four times srv. tne :•a'-

conditions used to obtain the top image. High res<u n ri 1mg

of this region, which are not shown here, indicate e

defect structures shown in the top image enlarge ani e

grow together to form much larger defects as a

substrate interactions. The bottom part. of Fgre

second image of the same region after imaging the center part four

times. Three aspects of the bottom micrograph are worth noting.

First, the center-most region of the organic surface has been

etched by the STM tip. Second, there is evidence that. the organi'c

material removed from the center part of the scan has been

deposited on the left and right edges of the etched feature.

Third, the small defects surrounding the etched portion of the

surface have been enlarged somewhat even though they have only

been exposed to the tip during two scans. These data clearly

demonstrate the feasibility of using tip-substrate interactions to

create high-resolution, nanometer-scale features on surfaces.

The physical basis for STM-tip-induced lithography is not

certain at the present time, but some combination of the following

four phenomena seems likely to contribute: (1) electron-beam-

induced degradation or desorption; (2) field ionization of

molecules resident near the tip-substrate gap that are accelerated

towards the surface, thereby desorbing the resist; (3) physical

-7-

abrasion of the Au surface by the tip; A) Joue •t:nc of the

substrate, arising from current. flowing throghth- gap

resistance, and subsequent thermal desorpt~on oi the res The

fundamental problem associated with assignIcg the pxticuliar t1.

substrate interaction(s) responsible for monolayer removal ar:;

from the uncertain z-axis displacemrnnt of the tip reiarive to the

substrate during imaging. Data discussed later suggest the tip

might be 1-2 nm above the Au surface dur-ing imaging, but since S":'ý

images are a convolution of tunneling proability and topography,

this estimate is quite speculative. Morecoer, since each STM s11 n

removes some of the organic material from the surface, it is

likely that z-displacement is also a function of the number of

times the surface is scanned and, of course, the bias voltage and

tunneling current employed. Based on the data shown in Figure 1,

particularly the distribution of organic material in the bottom

image, our present hypothesis focuses on physica1 abrasion of the

surface as the dominant tip-substrate interaction responsible for

STM-induced nanolithography under the conditions employed in these

experiments.

Figure 2A shows three 60 nm x 60 nm STEI-defined features

Figure 2

confined to a single Au(lll) terrace. In contrast to the

incomplete etching of the structure shown in Figure 1, all of the

organic material appears to have been removed within these etched

regions. This results from the more vigorous etching conditions

used to fabricate the str.c.ures con nFiu

with the tip biased at +3 v and a , ,t)

rate = 31 25 Hz) followed by four scans a'

current and scan rate. The first set of scna ipn

most of the rmnnolayer resis, bt h m':,;• .

completely remove the organic, maerial trý:-.,h,

etchea features. We have been able to c r:eae q:-:we

defined structures similar to those shown in rFgue 1'

small as 25 x 25 nm, and it appears that thI- imio

is determined only by th, size of the tIp an-, . di"

resist molecules. Such structures are d "c .. ..

least several days.

Line scans corresponding to the data in pl.ur,. 2tA are

in Figure 2B. We intentionally scanned over an atomic st< as

internal calibration of topography in acquir:ng , -a .. t'. Thi

feature can be seen in line scans 1-3, and it clearly :ndzcates

that the morphology of the underlying Au substrate can be reiably

imaged through the organic monolayer: the line scans indicate that

the Au step height is 0.22 nm, which is close t toe-e O.2.4 nm

interlayer spacing of Au(lll)912

The most interesting aspect of the 1ine scans relates to the

"depth" .f the etched features. FTIR and ellipsometric data

indicate the height of HS(CH?)j-7CH3 monolayers rance from about 2.2

nm to about 2.8 nm, but the line scans shown in Figure 2B

reproducibly indicate a depth of only 0.7±0.1 nm.>-4

Electrochemical data discussed later strongly suggest., but do not

unambiguously prove, that most of the organic resist material has

-9-

been removed from the bottom of t.e etched features. :f n&he

bottoms of the pits are clean, and if thre Au it•elf Vs not to"!.

etched, the STM tip is probably within, ranher than arove, the L,

-alkanethiol monolayer during imaguing. Kim and B":! repcrt&ed rn

depth of somewhat smaller STM-induced defects to tQ I.+-. nm,

and they explained this anomalous behavior as resulting from

differences in tunneling probabilitves.9b while ou:r data support

these previous results, we do not believe it is possible to

confirm either model at the present time.

Since it has been well-established that STM images of organic

surfaces are difficult to unambiguously interpret, 0 we used

electrochemical methods to confirm the STM data. Additionally,

the data presented here clearly indicate that STM lithography is a

good means for fabricating ultramicroelectrodes. The

electrochemical experiment is illustrated in Scheme T, and the

Scheme I

details of electrode fabrication were discussed in the

experimental section.

Figure 3A is the cyclic voltammetric response to a 5 mIL

Figure 3

Ru(NH3 )6 3 ÷ solution obtained for a Au(lll) facet after masking with

silicone rubber, but prior to HSICH2 )17 CH3 modification. The data

are intermediate between those expected for linear and radial

diffusion: a consequence of the small size of the Au(lll) facet.

-10-

After monon <ayet mw.if,"a i . Fi .gu- -i j t:,.

Faradaic current is . :... " .. -

combination of Fat ±•-A.sJi 1 ;i>n .that

advent1tious defa,; a in -v no ,_n.. ,z -:. , ;:. .

and tunnelingq c:urrer.: : V h "

Ru (NH3) 6-* . T~he C<y<'l,, Vo'j•, =r ,: a:•',- tv ,_ ,,,,-: : - .: -- • . , ,•

STM tip-inducel-: etching otne4 n i:w w.

HS(CH2) 17CH3 monoiayer t"S=,:; . A .A -a..- I',!-•V . -. :;1:

such a window is Mhoiwn; in • ". • A:

Figure 4

fabricated by scanning• he u:,-f07- twi." with o !;at v.1' 01:,

V, a tunneling current -. Ic .l , an! a -- t .* M,

The sigmoidal shape of the ... v ar,-.... : ..

radial diffusion to a small , e:. . d. in ... 7.,'thin -iA- .. . .. .. ....

ultramicroelect rode. We can ':use ,:,o-; t• i-

value for the limiting vomitat- currnt .j if wr7-1- .. -,

electrode as a disk of ra-i-A" r, rather rn ars

For this experiment. n 1 i equivmoi, F = 96485 Clequ.v, D ... . X

10-6 cm 2 /s, 1 6 and C - 5 0 x l.- m-"fcmt Approximn"i;. i by

subtracting the residual current at -0.38 7 in F1g3urc,• 32 from -he

current in Figure 3C, we calculate a value of r = 3.6 Vm. We

believe the difference between this value and the ,le ob .ai nd Y,

-ii-

visual inuspec~t•. t~ f <::-4 * -" P' • ,, :.::i: :

our assurnp " ,cn . -.. - ....

main po int is L ,•: ~ "' "': : • : ,'i.. :, ° .- •L -' ,.?, : .;

d a t a c o n f i tm tr h e .t.:'K

three additrija 5 x u. ...

in the con figurir oc ':I1' . 4 . . . .

results in an array of ult.,...t

abou threet h re•e t 'me e a rqt,, •

expected the " i ' r"j ..... : ... .: ,

the four ele r..rodtu- ,.:, ' , -. ': ' I,.:.

overlap. z

A plot of £ vers.x3 iog i-; -. , .. ... . ::::

part of the voltam~rogriims shown in Fqures 3C Jnd y: y :d a

slope of 90 mY, close to the Ialue for a th:,rdo. -i:i-y

reversible one-electron tr.ns r - 2: < n, .

supports, but does not .ncim....- .. ..- nim,

most of the organic material has been removed from 7h, . -c ,

surface in the etched areas, since a signlficant :ns'].3tIncg iayer-

would further reduce the rate of electron transfer between the

electrode and Ru(NH3 ) 63 ÷ and thus dec.ease the so t

part of -he cyclic voltamrmogram. Figure 4 suqgests the presence

of some organic material wit.hin etched region, butr the

distribution is such thca the effect on the elect iochemical

response should be negligible. The three important points that

result from the data shown in Figures 3 and 4 are: (1) it is

possible to use the STM tip to nanolithographically define

-12-

electrode arrays; K> -td i< z.LK I

result in additional F~r d.ý:u t.. ..

(3) since the elec, r Pr; .... -s : :, . .: -,

material has been reAv4d :r. tLe A• . , .'; - ;

the etched fea1Aure%.

This preliminaryrert rvi.;h! :v

intentional ST-M fabricatio:r of we:!-d•,ei f,,•,,: -:". "

self-assembled monolayer resist. The es:en.i2i t•' .<t

STM images of the nanolithographically-dle ined wrds; :.r,

confirmed by companion electrochernical experients jdri .cann!r,%

electron micrographs. Combination of the three method•s:. yields

complementary information about the nature of the pat_.te.rns• , and ,

also provides a new means for studying the nature of -the t*ip

monolayer interaction. Perhaps the most important aspect of th'i

work is that the monolayer resists are sufficiently thin to permirt

electron tunneling, but sufficiently thick to effectively block

significant mass transfer and Faradaic electron transfer across

them.

At the present time, we are pursuing several aspects of the

preliminary data presented here. First, we are using low

temperature chemical vapor deposition techniques to selectively

metallize etched regions. 17 Second, we are trying to improve the

blocking quality of the monolayers so that we can construct very

well-defined ultramicroelectrodes in the size range 1-10 nm; at

-13-

the present time elecrrodie. wii! .. cr•.> . a :-:;•,:.: , Lu:.

100 nm are too leaky to be :a-u :

experiments designed to eIucidate t..h mecL<rnasm :e:.... ...• I'.2•

tip-induced lithography. PreIimi~nary results fron th£-e

experiments will be reported shorti-y.

We gratefully acknowledge the Office of Naval Reserch for full

support of this work.

-14-

REkLr El ES

1. The results discussed here have previou:siy bee% presne

orally. (a) Crooks, R. M.; Sun, L.; Thcmlas, R.;

0. Abstracts of Papers, 18th Annual Meeting ct tc14: edeia>:.

of Analytical Chemistry and Spectroscopy Societi

CA; Federation of Analytical Chemistry and Spectroscopy

Societies: Leavenworth, KS, 1991; Abstract 657. (b) Crooks, F.

M.; Ross, C. B.; Sun, L. Abstracts of Papers, 182nd Naticnal

Meeting of the Electrochemical Society, Toronto, Canada, 1992;

Abstract 629.

2. Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481.

3. Bain, C. D.; Troughton, E. B.; Tao, Y-T.; Eva!!, J.;

Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111.

321 and references therein.

4. Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43,

437, and references therein.

5. Wolf, S.; Tauber, R. N. Silicon Processing for the VLSI Era;

Lattice: Sunset Beach, CA, 1986; Chapter 14.

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Vac. Sci. Technol. B. 1988, 6, 1861. (b) Randall, J. N.; Reed,

M. A.; Moore, T. M.; Matyi, R. J.; Lee, J. W. J. Vac. Sci.

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7. Binnig, G.; Rohrer, H.; Gerber, Ch.; Weibel, E. Phys. Rev.

Lett. 1982, 49, 57.

8. The following references are a brief but representative cross

section of the scanning probe modification literature: (a)

-15-

Ringger, M. ; Hidber, H. R. ;hioii,

Gantherodt, H.-J. A~pI. Phys. Let:. 1985, *•, 1K> 1> uiu:o.

U. ; Wiesendanger, R. ; L.g ,o,- r, . - H -

GQntherodt, H.-J. ; Garcia, N. App, .... 1987,.

(c) Lin, C. W. ; Fan, F-R. ; Bard, A. J .

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Staufer, U.; Wiesendanger, R.; Eng, L.; Rosen1hal(: i,

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Brumfield, J. C. ; irelrie, E. A. 1992,

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R. C Llangmui: 1992, , &> 281f]). (t

P.; Whitesides, G. M. Science 1992, 25,- ,389.

9. There have been two prior paper:•. that. ar•e a-t : ! ,.

to the results described here: ) Cas'lýaa .

an STI4 tip could be used to etch .e -, u .

thin layer of Pt-confined Ti,'." Cas,"a- !,l.;

White, H. S. 31. Elec!rocnem "-c c. 1991, 4 1.

Bard demonstrated that STM imagingj of n-acika-evt.ho mno1m ,.n r .

results in lateral expansion of advenititious der:cts {Kim, Y. -

T.; Bard, A. J. Langmuir 1992, 8, 1096).

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C. R.; Beebe, T. P., Jr. Science 1991, 251, 640.

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14. Trevor, D. J.; Chidsey, C. E. D.; Loiacono, D. N. Phys. Rev.

Lett. 1989, 62, 929.

15. Wightman, R. M.; Wipf, D. 0. In Electroanalytical Chemistry,

Bard, A. J., Ed.; Marcel Dekker: NY, 1989; Vol. 15, p. 267.

16. Licht, S.; Cammarata, V.; Wrighton, M. S. J. Phys. Chem. 1990,

94, 6133.

17. (a) Shin, H. K.; Chi, K. M.; Farkas, J.; Hampden-Smith, M. J.;

Kodas, T. T.; Duesler, E. N. Inorg. Chem. 1992, 31, 424. (b)

Shin, H. K.; Chi, K. M.; Hampden-Smith, M. J.; Kodas, T. T.;

Farr, J. D.; Paffett, M. Chem. Mater. 1992, 4, 788.

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1. STM images of a Au (lil sur face mod"eed with a rzo ýoa,

HS (CH2 ) 17CH3. (Top) First 400 x 400 nim scan. (Bottom) Second 4n

x 400 nm scan. Four scans of the center 00 x 100 nm rgon

the surface were obtained prior to recording of the bottom itao,-.

Conditions for all six scans: bias voltage = +300 mV; tunneling

current = 0.10 nA; scan rate = 1.34 Hz.

2. (A) STM image of a Au(!lll substu&te modified with a monolayer

of HS(CH2 )I7CH 3 after opening three 60 x 60 nm windows. STM

etching conditions: four scans (bias voltage = +3 V; tunneling

current = 0.11 nA; scan rate = 31.25 Hz) followed by four

additional scans (bias voltage = +300 mV; tunneling current 01.11

nA; scan rate = 31.25 Hz). (B) STM line scans through the etchpd

regions which are shown in (A) and illustrated schematically to

the right of the line scans. The vertical displacement (v.d.)

between the arrows is indicated next to each line scan.

3. Electrochemical results obtained at (A) a naked Au(lll) facet;

(B) the same facet after modification with a monolayer of

HS(CH2 ) 1 7 CH 3 ; (C) the same facet after STM fabrication of a single

5 x 5 gm ultramicroelectrode; (D) the same facet after fabricating

four 5 x 5 pm ultramicroelectrodes spaced as indicated

schematically to the left. Conditions: solution, 5 mM

Ru(NH3 ) 63 ÷/0.l M KCl; scan rate = 100 mV/s.

-18-

4. Scanning electron mincrogriph ,-) cý

definied ultramicroelectrodec.

-19--

tac6.~, "Alta;

r) a-c a.A

Line Scans Top View

SAu Step

2

3

iJ 200 300 400 nm

Figure 2/Ross et at.

Substrate Modification

n-alkaneth.-I<-monolayer

glue

STM Lithography

STM tip

Electrochemical Anayi

Scheme I -Ross et. al.

i (nA) 50

0-

A. Naked Au (111)Facet -50-

0 -100

-0.200.

B. Au (111) I i (nA) 5HS(CH 2)1 7CH 3 z.....

-0.2 0 0.ý2

I (nA) 5-

0-

C.~' One5 x5 g

windows

-15

-0.2 6 0. 1220Opm E (V) vs. Ag /AgCI

Figure 3/Ross et al.

TEC•ItCAL. REPORT DrsT•r:':IBs C:TW

office of Naval Research (2) Dr. Richtard !. W r. z(zChemistry Division, Code 1113 Naval Civil E:gleecn800 North Quincy Street LaboratoryvArlington, Virginia 22217-5000 Code L52

Port Hueneme, CA 92-43

Dr. James S. Murday (1)Chemistry Division, Code 6100 Dr. Harold H. S.inerzan (1'Naval Research Laboratory Naval Surface Warfere CenterWashington, D.C. 20375-5000 Carderock Divisic- -etac7eznt

Annapolis, MD 21402-1198

Dr. Robert Green, Director (1)Chemistry Division, Code 385 Dr. Eugene C. Fischer (1)Naval Air Weapons Center Code 2840

Weapons Division Naval Surface Warfare CenterChina Lake, CA 93555-6001 Carderock Division Detachnent

Annapolis, MD 21402-1198

Dr. Elek Lindner (1)Naval Command, Control and Ocean Defense Technical Information

Surveillance Center Center (2)RDT&E Division Building 5, Cameron StationSan Diego, CA 92152-5000 Alexandria, VA 22314

Dr. Bernard E. Douda (1)Crane DivisionNaval Surface Warfare CenterCrane, Indiana 47522-5000

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